The long-term goals of this project are to understand the concurrent development of the acoustical cues used to determine the spatial location of sound sources and the neural circuits in the ascending auditory pathways that encode these cues. All acoustical cues for localization can be captured in measurements of head related transfer functions (HRTFs), which contain the three main localization cues, interaural differences in time (ITDs) and level (ELDs) and monaural spectral cues. Since the formation of the cues is governed by the linear dimensions of the head and pinnae, the relative magnitudes of the cues will change with increasing growth of the head and pinnAe during development. The proposed experiments comprise two specific aims designed to test the hypothesis that over the course of the development of one of the acoustical cues for sound localization, ILDs, there is a concomitant development of the ability of single cells in the lateral superior olive (LSO), one of the first sites in the ascending auditory pathway for binaural interaction, to encode ILDs. Specific Aim I will test the hypothesis that the values of the localization cues will change in ways predictable from the increasing dimensions of the head and pinnae during development. To accomplish this, HRTFs for spatial locations in the horizontal and vertical planes in the frontal hemisphere in kittens aged from approximately 1 week postnatal to adult as well as the physical dimensions of the head and pinnae will be measured. Specific Aim II will test the hypothesis that the range of ILDs encoded by LSO cells will increase during development with the expected increase in the magnitudes of the physical ILDs, as measured in Aim I. The development of the ability of single cells in the LSO to encode ILDs and the ability of cells in the medial nucleus of the trapezoid body, which provide the inhibitory input to the LSO, to encode sound level will be studied. These studies will determine if and how long the nuclei in the ascending binaural auditory system can compensate for changes in the acoustical environment brought about by developmental changes in the acoustical properties of the head and pinnae.